1. Field of the Invention
The present invention relates to an electron gun in a cathode ray tube and more particularly to a triode portion of an electron gun that allows adjustment of a beam cross-over point and improves cut-off level.
2. Description of the Related Art
A cathode ray tube displays an image by scanning electron beams emitted from an electron gun to the phosphor-coated screen of an evacuated tube. Conventionally, an electron gun is comprised of a triode, a plurality of focus electrodes and a final accelerating electrode, arranged in sequence. The plurality of focus electrodes form a pre-focus electron lens while the final accelerating electrode along with its adjacent focus electrode form a main lens. In an in-line aperture electron gun, the electron beam spot becomes undesirably large and distorted due to a non-uniform deflection field generated by a deflection yoke, which makes the focused electron beams deflected to scan the screen. In order to prevent such distortion of electron beam spots, a dynamic focus quadrupole lens electron gun is commonly used, which distorts the beam spots in such a manner that the distortion due to the deflection yoke is canceled and changes a focus voltage when the periphery of the screen is scanned. Referring to FIG. 1, a conventional triode consists of three cathodes 11, a control electrode 12 and a screen electrode 13. On the control electrode 12 and screen electrode 13 are formed three in line electron-beam passing holes 14 and 15 corresponding to the three cathodes 11. The apertures 14 in the control electrode 12 are vertically elongated slots while the apertures 15 in the screen electrode 13 are horizontally elongated slots. Electrons emitted from the cathodes 11 are pre-focused as they pass through the apertures in the control and screen electrodes 12 and 13. The thicknesses of the electrodes 12 and 13 and the shapes of the apertures 14,15 affect the beam incident angle to the main lens. As the control electrode 12 is made thinner, the size of the beam spots becomes smaller, which results in a smaller beam incident to the main lens so that the spherical aberration is reduced. A thicker screen electrode 13 also reduces a beam incident angle to the main lens so that the electron beams are focused more intensely. The sizes and shapes of apertures 14, 15 of the electrodes 12 and 13 are also a factor in the determination of the electron beams focusing. If apertures 14 in the control electrode 12 are large with all the other things being equal, more beam current can flow so that the beam spots become larger. On the other hand, if the apertures 14 are small, the current density must increase in order to maintain a same brightness level. In the other words, a larger quantity of electrons should be produced. In this case, the cathodes 11 do not last long. When the apertures 15 in the screen electrode 13 are smaller, the pre-focusing gets stronger and accordingly the incident angle to the main lens becomes smaller. Therefore, the electron beams experience less spherical aberration. However, the beam spots become larger because the beams are under the influence of a weaker main focusing. The sizes of the screen electrode apertures 15 are related to the cut-off. Large apertures 15 allow an electric field to penetrate into the first focus lens"" electric field. This has an effect of increased electric field on the surfaces of the cathodes 11 but that of the screen electrode 13 decreases. Thus, the influence of the aperture size on the cut-off voltage is not that serious. If the distance between the control electrode 12 and screen electrode 13 is large, the lens becomes stronger and incidence angle in the cross over becomes smaller. On the other hand, if the distance between the control electrode and the screen electrode 12 and 13 is small, a defocusing phenomenon of electron beams deflected on the periphery of the screen can be reduced. However, the beam spot size in a center area of the screen increases such that the resolution becomes poor.
It is an objective of the present invention to provide a screen electrode having electron beam passing apertures that allows the adjustment of a cross-over point and improves the cut-off level characteristics ultimately for reduced spherical aberration. In order to achieve the objective, the screen electrode of an electron gun for use in a cathode ray tube has multi-stage apertures such that the entrance area of an aperture is larger than the exit area thereof. This has an effect of smaller screen electrode apertures with a result of increased pre-focusing of electron beams passing through the apertures. Thus, increased pre-focusing reduces the beam incident angle to the main lens. A smaller beam incident angle generates less spherical aberration in the main lens.